The control of infection by plant pathogens, which can inhibit production of fruits, seeds, foliage and flowers and cause reductions in the quality and quantity of the harvested crops, is of significant economic importance. Pathogens annually cause billions of dollars in damage to crops worldwide (Baker et al. 1997, Science 276:726-733). Consequently, an increasing amount of research has been dedicated to developing novel methods for controlling plant diseases. Such studies have centered on the plant's innate ability to resist pathogen invasion in an effort to buttress the plant's own defenses to counter pathogen attacks (Staskawicz et al. 1995, Science 268:661-667; Baker et al. supra).
Although most crops are treated with agricultural pesticidal agents, such as anti-fungal and anti-bacterial agents, damage from pathogenic infection still results in revenue losses to the agricultural industry on a regular basis. Furthermore, many of the agents used to control such infection or infestation cause adverse side effects to the plant and/or to the environment. Plants with enhanced resistance to infection by pathogens would decrease or eliminate the need for application of chemical pesticidal, anti-fungal and anti-bacterial agents. There has been significant interest in developing transgenic plants that show increased resistance to a broad range of pathogens (Atkinson et al., 2003, Annu. Rev. Phytopathol. 41:615-639; Williamson and Gleason, 2003, Curr. Opin. Plant Biol, 6:327-333; Stuiver and Custers, 2001, Nature 411:865-8; Melchers and Stuiver, 2000, Curr. Opin. Plant Biol 3:147-152; Rommens and Kishore, 2000, Curr. Opin. Biotechnol. 11:120-125; Williamson, 1999, Curr. Opin. Plant Biol 2:327-331; Mourgues et al 1998, Trends Biotechnol 16:203-210).
Plant pathogenic nematodes are small invertebrate animals that feed on the roots of crops causing damage to the plants and reducing yield of the crops. Nematodes of the family Heteroderidae cause the most economic damage among plant parasitic nematodes (Williamson, 1999, Curr. Opin. Plant Biol 2:327-331). This family of parasitic nematodes can be divided into two groups: the root-knot nematodes (genus Meloidogyne) and the cyst nematodes (genera Heterodera and Globodera). Infection of host plants by the root-knot nematodes usually results in the formation of root galls or ‘root-knots’, and causes severe loss in yield in many crops. By contrast, cyst nematodes often have narrower host ranges. Arabidopsis thaliana, which is amendable to molecular genetics experiments, is an important model for providing insights into plant-nematode interactions because it is a host for several species of root-knot and cyst nematodes (Sijmons et al., 1991, Plant J., 1:245-254).
A number of genes whose mis-expression is associated with altered resistance to nematodes have been identified in several crop species. For examples, the Mi gene of tomato confers resistance against several root-knot nematode species (Williamson, 1998, Annu. Rev. Phytopathol 36:277-293). Mi protein contains NBS (nucleotide binding site) and LRR (leucine rich repeats) domains (Kaloshian et al., 1998, Mol. Gen. Genet., 257:376-385; Milligan et al., 1998, Plant Cell 10:1307-1319). The Hs1pro-1 gene of a wild relative of sugar beet confers resistance to the cyst nematode Heterodera schachtii (Cai et al., 1997, Science, 275:832-834). Hs1pro-1 protein contains a predicted signal sequence, a predicted trans-membrane region and a leucine-rich region. The Gpa2 gene of potato confers resistance against some isolates of the cyst nematode Globodera pallida (van der Voort et al., 1999, Mol. Plant-Microbe Int., 12:187-206; van der Vossen, 2000, Plant J., 23:567-576). The Hero gene of tomato confers resistance to potato cyst nematodes such as Globodera rostochiensis and G. pallida (Ernst et al., 2002, Plant J., 31:127-136). The Gpa2 and Hero proteins, similar to the Mi protein, contain the NBS and LRR domains. Lastly, the Cre1 gene of wheat confers resistance to most European nematodes and the only Australian pathotype; whereas the Cre3 gene of wheat confers resistance to the Australian nematodes (de Majnik J et al., 2003, Mol. Plant Microbe Interact. 16:1129-1134). The Cre1 and Cre3 genes have not been cloned.
Due to the importance of pathogen resistance in plants, methods for producing plants with increased pathogen resistance are desirable.